November, 1944 



Bellrose: Duck Populations and Kill 



335 



quotient is given to the ratio that exists optimum dates for the hunting season, 



between the percentage a species con- This relationship is expressed by a 



tributes to the duck bag and the per- mortality quotient^ which is similar to the 



centage that species makes of the duck vulnerability quotient except that it 



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— « WATERFOWL FLIGHT 



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1939 



HUNTING SEASON 



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Fig. 4. — Chronology of flight of all duck species, and species other than mallard, in relation 

 to the hunting season in the Illinois River valley, 1939. 



flight during the opeit seasori. For ex- 

 ample, the pintail, which for a 5-year 

 period contributed 9.65 per cent of the 

 total Illinois bag, comprised 1.17 per 

 cent of the total Illinois flight during the 

 open season. The vulnerability quotient 

 jfor this period is 9.65 divided by 1.17, 

 or 8.25, table 2. This quotient, when 

 icompared with vulnerability quotients 

 ifor other ducks, may be considered as a 

 relative measure of the killability of the 

 ipintail, or the relative ease or difficulty 

 iwith which it is bagged. 

 I The relationship of the bag for any 

 species to the entire Jail flight of that 

 ppecies also is necessary in determining 



takes into account a longer period of 

 migration, tables 2-7. It is a measure 

 of the shooting pressure placed upon a 

 species by its vulnerability and its chro- 

 nology of migration in relation to the 

 hunting season. 



The comparative influence on each 

 principal duck species of changes in the 

 hunting season may be obtained by 

 finding the ratio of the mortality quo- 

 tient to the vulnerability quotient. A 

 numerical expression of this relation- 

 ship, obtained by dividing the mortality 

 quotient by the vulnerability quotient, 

 is termed the shooting pressure quotient, 

 tables 2 and 8. In this figure, vulner- 



